From
Letters of J.R.R. Tolkien,
letter 211:
I could have fitted things in with greater versimilitude, if the story had not become too far developed, before the question ever occurred to me. I doubt if there would have been much gain; and I hope the, evidently long but undefined, gap* in time between the Fall of
Barad-dûr and our Days is sufficient for 'literary credibility', even for readers acquainted with what is known or surmised of 'pre-history'.
* I imagine the gap to be about 6000 years : that is we are now at the end of the Fifth Age, if the
Ages were of about the same length as S.A. and T.A. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh.
Of course this really doesn't fit very well.
I think
Tolkien, knowing that the first interpretable written texts date to sometime after 3000 BCE, that is 5000 years ago, just added another thousand years to bring the history back to the fall of Barad-dûr.
The real problem is the change in the geography of Europe, a change involving not only the coastline but entire
mountain ranges. Tolkien never explained how this great change came about and we are probably not supposed to ask.
He notes he really did not fit in the geography properly. But then, even if he had done so, one still could not actually go to the
places where he would have placed
Minas Tirith, for example, and expect to be able to excavate them.
An impossible geography works well enough.
However somewhere in the HoME series Tolkien refers to at least one ice age following the fall of
Númenor.
That would put his three Ages even farther back, and maybe increase the plausibility of something like the story actually occurring.
Since the Ages are ended by great
events, if Tolkien thinks we are either at the end of the Sixth Age, or in the Seventh, we might ask what events Tolkien thinks might have marked off these later Ages.
Probably either the birth or resurrection of Christ would be one point of demarcation.
But Tolkien probably did not want to make some things explicit to readers or to himself.